Went • The Role of the Environment in Plant Growth 



101 



contributes to variability had not been 

 properly realized, but experiments 

 make it clear that most biological vari- 

 ability in genetically uniform material 

 was due to uncontrolled environment 

 during growth. Whereas in an ordi- 

 nar)' greenhouse the coefficient of vari- 

 ability of, e.g., tomato plants may be 

 20%, under complete temperature and 

 light control this is reduced to below 

 5%. Therefore at least 80% of pheno- 

 typie variability may be due to uncon- 

 trolled environment. . . . 



When, instead of growing plants 

 under exactly similar environmental 

 conditions, we subject them to different 

 temperatures, light treatments, wind, 

 or other climatic variables, then an 

 amazing degree of variability in growth 

 rate, yield, form, flowering and fruit- 

 ing behavior, and chemical composi- 

 tion can be produced. This is a com- 

 pletely different aspect of the Earhart 

 Plant Research Laboraton', and this is 

 the basis on which it can be used to 

 study the effects of climate on plants. 

 Each of the important climatic factors 

 can be investigated separately or in con- 

 junction with one or more of the others 

 in relation to growth and development 

 of the different plant. In this way it 

 can be established which factors are 

 most important in the life of a plant, 

 and it can be judged how climate as a 

 whole may affect a plant. 



Almost any character can be modi- 

 fied by one or more of the individual 

 climatic factors, and often various 

 characters are changed in the same di- 

 rection. In the tomato, for instance, 

 both growth in length, increase in 

 weight, and fruit production are opti- 

 mal when the night temperature is 

 17° C. But heaviest stems and largest 

 leaves are produced at slightly lower 

 night temperatures. At high night tem- 

 peratures flower size and fruit develop- 

 ment are more inhibited than vegeta- 

 tive growth. In potatoes it also is night 

 temperature which mainly controls 



tuber formation. This is optimal at 12° 

 C. and is relatively little affected by 

 the temperature during day. In peas 

 and strawberries the day temperature 

 is more important for growth and fruit 

 set than the night temperature. In 

 peaches and in Veratrum normal de- 

 velopment is possible only when there 

 is a seasonal fluctuation in tempera- 

 ture. Thus for each species and variety 

 of plant there is a parricular climatic 

 factor or set of factors which primarily 

 controls development and growth. 

 Once these factors have been estab- 

 lished in the laborator\' it becomes pos- 

 sible to interpret field observations in- 

 telligently. As an example it can be 

 mentioned that once the significance 

 of the night temperature for fruit set 

 in tomatoes had been recognized 

 through experiments in air-conditioned 

 greenhouses, it became possible to ex- 

 plain poor tomato production in local- 

 ities with too high or too low night 

 temperatures, or in years with excep- 

 tional weather. 



Since each climatic factor — day tem- 

 perature, night temperature, light in- 

 tensity, light duration, spectral compo- 

 sition of the light, relative humidity, 

 wind, rain, seasonal fluctuation in tem- 

 perature, gas content of the air, frost, 

 etc.— is an independent variable, and 

 since there can be any conceivable in- 

 teraction in plant response between 

 these factors, the effect of climate on 

 a plant cannot possibly be expressed 

 in a simple formula, like a "heat sum." 

 We are dealing here with multi-dimen- 

 sional interrelationships, which can 

 only be expressed in multi-dimensional 

 diagrams. Actually the optima of some 

 plants are so different that they cannot 

 be grown at all. The optimal tempera- 

 tures for the African violet are so high 

 that the English Daisy dies in them, 

 whereas the African Violet dies under 

 the optimal growing conditions for the 

 English Daisy. Unfortunately so httle 

 is known about this subject as yet. . , . 



